The impressive cloud in the region of the star rho Ophiuchi is one of the heavenly meeting points for astronomers in search of young stars. Located 540 light-years away in the constellation of Ophiucus, in the celestial equator, the dusty clouds close to rho Ophiuchi are the nest of more than one hundred newborn stars. But ESA's Infrared Space Observatory, ISO, has also found a surprise hidden in the dust: 30 brown dwarfs, elusive and ambiguous objects considered to be 'failed stars' because they have too little mass to shine as stars. Relatively few of these brown dwarfs have been identified so far, so finding one is like winning a trophy. With this discovery ISO has turned the rho Ophiuchi region into a favourite game reserve for brown-dwarf hunters.

Three of the most massive young stars in this stellar nursery are easy to find in this image: one in the centre of the right-hand-side border; a second one in the middle of a comet-shaped nebula in the lower-right of the image; and finally, one in the middle of the small nebula close to the centre-right. Other point-like sources are also young stars and 'protostars' - stars that are still 'growing' by 'sucking in' gas from the cloud. Credit: ESA ISO/ ISOCAM/ Alain Abergel (Abergel et al. (1996) A&A 315, L329)

"ISO gives us a new, really rich sample of young brown dwarfs in the rho Ophiuchi region. We will clearly have to go back and search for more of these sub-stellar objects with current and future infrared telescopes, both in space and from the ground with the 10-metre class telescopes," says Sylvain Bontemps (Observatoire de Bordeaux, Floirac, France), a member of an international team led by Lennart Nordh (Stockholm Observatory, Sweden) that observed the rho Ophiuchi cloud with ISO.

Brown dwarfs are elusive because they are very faint, and ambiguous because their true nature is still unclear. Some astronomers say that at least some of them, the less massive ones, could be better described as giant planets, like Jupiter, instead of as failed stars. The minimum mass for a star to shine as such is 8 per cent of the mass of the Sun, or 80 times the mass of Jupiter - below that limit, the 'nuclear oven' that provides the star's energy cannot be ignited at the star's core.

In the case of the brown dwarfs found in the rho Ophiuchi region, "the less massive are about 5 per cent of the mass of the Sun, or 50 Jupiter masses. But certainly there could still be less massive objects hidden in the dust," Bontemps says.

This brown dwarf population has the added value of its youth. They are typically a million years old, and as a consequence they are still relatively bright. This makes them easier to study than other older brown dwarfs, whose light is weakened due to their very cold atmosphere.

ISO performed similar surveys in other nearby regions of star formation, such as Chamaeleon I and Serpens, which have also revealed the presence of young brown dwarf populations. All these results contribute to solving the question of what the true nature of brown dwarfs is.

This note is based on the results published in the scientific paper "ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster" by S. Bontemps et al., published in Astronomy & Astrophysics 372, 173, 2001.

The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998. As an unprecedented observatory for infrared astronomy ISO made nearly 30 000 scientific observations.